bundled or layered top bars at support in beam
bundled or layered top bars at support in beam
(OP)
The design of top bars at support of a 300mm width by 500mm depth rc beam has 7 bars. Is it recommended to put all 7 bars in one layer (top most) and bundled together in two's or distributed in two layers? What do you usually do in your practice? I think distributing it in two layers can diminish the moment capacity but with the advantage of less congestion.






RE: bundled or layered top bars at support in beam
Any larger bars, stack 'em and compute the loss. Probably one extra bar will suffice making it two layers of four stacked.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: bundled or layered top bars at support in beam
BA
RE: bundled or layered top bars at support in beam
The bars are 20mm 7 pcs in 300mm width beam (not tee beam). If two layers are used, what is usually the separation between the two laters? I usually handle 2 to 3 bars only. Is the moment capacity percentage decrease of two layers significantly great in your experience?
RE: bundled or layered top bars at support in beam
Perhaps you should consider using a larger diameter bar. Four 25mm bars will fit nicely in a 300mm wide beam.
BA
RE: bundled or layered top bars at support in beam
Maybe the 20mm can just be bundled two together up-down or left right and put in top most position and this is better than having 25mm separation between the top and next lower layer. Have you done this bundling in practice and why not if you do not?
RE: bundled or layered top bars at support in beam
BA
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
2 X 1.5" clearance = 3"
2 X .375" (#3) stirrups = .75"
7 X .5" (#4) diameter bars = 3.5"
Sum = 7.25"
Net available width = 4.56"
Divided among 6 spaces = .76" between each bar (1.5X the bar diameter but 1" minimum spacing is required between the bars - I forgot that...)
BA is correct here. Stack the bars...
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: bundled or layered top bars at support in beam
Mike, why convert? Sticking with mm is so much easier.
RE: bundled or layered top bars at support in beam
:)
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: bundled or layered top bars at support in beam
Bundled bars were not permitted when I started practicing engineering. Can't remember when bundling was introduced, somewhere around 1980 I think. Never did agree with the concept. Bond between steel and concrete, I believe, is seriously compromised by bundling. In any event, there is no need to do it so why bother?
I have always found when on site, that the concrete placers were not too happy even when I used the spacing of bars permitted by code. There is no point fighting them. Give them plenty of room to pour and vibrate the concrete. There seems to be no problem with one bar for every 4" (100mm) of beam width, so that is what I try to stick to. But in your case, I would go to 4-25M bars.
BA
RE: bundled or layered top bars at support in beam
Are you saying even in columns, bundled bars seriously compromised bonding between bars and concrete? or is it only in beams? I commonly bundled the corners of a column
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
Why is bundled bars in columns not a problem of bonding between bars and concrete while in beams it is a problem?
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
So the problem only occurs in the spliced sections of bundled bars? 5 of the 7 bars are just extra top bars at support. It doesn't need to be spliced to anything. There is no splice in the beams because 10 meter bars are used. So here bundled bars in beams are not problem as far as connections between bars and concrete are concerned? Or do you still think bundled bars without splice is still not good, and why is that?
RE: bundled or layered top bars at support in beam
BA
RE: bundled or layered top bars at support in beam
of course a bar normally develope bond thru the rigged surfaces of the rebars and the concrete. We used 4000 psi concrete, a single bar with rigged surfaces can bond with it, as is normally and usually the case in general construction. What I can't understand is if the bars are bundled two together, let us say two 20mm. Why is the bond between it and concrete poorer? It is the same 4000 psi concrete and vibrated as concrete is poured to the form works.
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
Imagine a column and continuous beam horizontal on left and right side of the column. The middle of the left beam (and right beam) has secondary beam perpendicular to it (so the floor slabs would be only 4" in thickness). The top 7 bars at support at column came out after calculations of the moment requirements at the support. Would bundling the 7 bars in two would be good idea or can I just add another bar to make it 8 and stack it up and down with vertical distance of 25mm. But before I do this. I want to know how bundled bars become poorer in the concrete connection to the steel.
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
To keep it symmetrical, you could place 5-20M inside the beam and 1-20M in the slab on each side. Or you could place 3-20M inside the beam and 2-20M in the slab on each side. I prefer the second option because it provides better opportunity for placing and vibrating concrete around the bars.
BA
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
RE: bundled or layered top bars at support in beam
My beam is a t-beam since slabs are monolithically connected to it. But is it common to put the beam reinforcement outside in the sides (of the t-beam)? I never use this. There may be some performance effect or calculation inaccuracy using this method. Also remember the 7 bars have 2 as continuous 10 meter bars (left and right beams of column) and 5 pcs as extra continuous L/3 bars at supports (top bars). So you are saying the 5 pcs can be only 3 at the beam and 2 distributed in the sides? How does this affect the performance. How common do people do this?
RE: bundled or layered top bars at support in beam
I never use this. There may be some performance effect or calculation inaccuracy using this method. Not true. It is good practice. Some engineers include the temperature reinforcement in the slab for the width of the T-beam flange as part of the top reinforcement for the beam.
Also remember the 7 bars have 2 as continuous 10 meter bars (left and right beams of column) and 5 pcs as extra continuous L/3 bars at supports (top bars). Ten meters is a long bar to handle. You could use bars from each beam and lap them at the column. That gives you four top bars. You need three more, say one at center and one in the slab on each side.
So you are saying the 5 pcs can be only 3 at the beam and 2 distributed in the sides? How does this affect the performance. How common do people do this? Not very common. It is more common to lap the bars as described above.
BA
RE: bundled or layered top bars at support in beam
BARetired, you said earlier "Our code in Canada, A23.3 permits bundled bars but there are special requirements for developing the bar strength in a bundle". Can you just give a clue what's the special requirements?
By the way, when you lap bars, it has same appearance as bundled bars! So you don't bundle bars yet you lap bars and they look the same.
RE: bundled or layered top bars at support in beam
In CSA A23.3, tension lap splices require 30% more development length than that required to develop either bar. So if you know your theoretical cutoff point, you must add 1.3 times the necessary development length to each of the lapped bars.
BA
RE: bundled or layered top bars at support in beam
I'll choose this option of two layers. Only 4 bars can fit at the top of the 0.3m width due to the stirrups 10mm diameter and Four 20mm. Assume the depth of the beam is 0.5m. Putting the second layer 25mm below the first one is like having a beam depth combination (and moment capacity) of 0.5m and 0.475m, isn't it. Know any rule of thumb of knowing whether to adding one 20mm bar to the second layer is enough to compensate?
RE: bundled or layered top bars at support in beam
I seldom see bundled bars. The main focus for development is to be sure of proper consolidation and good concrete mix design. Using a polycarboxylate to help the paste flow and keep aggregates in suspension with more fluid concrete. The interlock of deformations on one bar to those on the adjacent bar work with the paste and with the surrounding concrete to develop the reinforcement.
With any very large bar or bundle (i.e., large area of steel) the surrounding concrete is asked to do quite a lot. The local compressive and tensile forces in the concrete can be quite high, including splitting forces.